1. Field of the Invention
The present invention relates to a brushless motor to be mounted in a data storage medium drive for a data storage medium such as an optical data storage medium. More particularly, the present invention relates to an improvement in a rotor retaining mechanism in association with a thickness reduction of the brushless motor.
2. Description of the Related Art
Strenuous efforts are being made to reduce the size and weight of personal computers, which are mainly used as portable computers, in order to improve portability thereof. Coupled therewith, thickness reduction to an utmost limit is required of the data storage medium drives used for storing and retrieving data stored in a data storage medium (e.g., optical data storage medium) mounted in the personal computers. Also, thickness reduction is required of the spindle motors (hereafter, motors) mounted in the data storage medium drives.
Conventionally, a ball chuck system is used as a method for holding a data storage medium in a data storage medium drive. The ball chuck system provides the data storage medium with a holding strength of about 300 gf or greater; hence, when a user pulls the data storage medium up in order to remove the medium from the spindle motor, the rotor of the spindle motor may be lifted together with the data storage medium. For this reason, the spindle motor includes a rotor retaining mechanism preventing the rotor from being separated from other components of the spindle motor.
Conventionally, the rotor retaining mechanism includes a shaft retaining type and a hook-shaped type.
A conventional retaining mechanism will be described with reference to FIGS. 9 and 10. FIGS. 9 and 10 show schematic cross-sectional views taken along an axial direction of spindle motors mounted in data storage medium drives. FIG. 9 shows a retaining mechanism of the shaft retaining type, and FIG. 10 shows a retaining mechanism of the hook-shaped type.
The common structure of the spindle motors shown in FIGS. 9 and 10 includes a rotor 100 on which a data storage medium such as a compact disk is mounted in a removable manner, a bearing portion 101 for rotatably supporting the rotor 100, and a housing 102.
The rotor 100 has a substantially circular disk-shaped turntable 103 for placing the data storage medium on its upper surface, a chucking device 104 fitted on the upper surface of the turntable 103 for removably holding the data storage medium, a shaft 105 attached integrally with the turntable 103, and a rotor magnet 106.
Inside the housing 102 that is attached on an attachment plate 107, the bearing portion 101 is fixed for rotatably supporting the shaft 105. A stator 108 is fixed at the upper outer side of the housing 102. The stator 108 is disposed opposite the rotor magnet 106 with a gap in between to produce a rotational magnetic field for rotatably driving the rotor 100. A pressurizing magnet 109 is disposed between the turntable 103 and the housing 102 provided with other components. The pressurizing magnet 109 magnetically attracts the turntable 103 to the side of the housing 102, so that the turntable 103 can be stably rotated.
The rotor 100 is mounted onto the housing 102 provided with other components with the shaft 105 inserted into the bearing portion 101, for example. In order to prevent the mounted rotor 100 from coming off, a rotor retaining mechanism is provided.
As shown in FIG. 9, a rotor retaining mechanism of the shaft retaining type is defined by a groove 110 encircling the whole circumference of a lower end portion of the shaft 105 of the rotor 100, and a retaining member 113 fixed on the housing 102 with its tip 111 positioned inside the groove 110. As the shaft 105 moves upward, the lower inner surface 114 in the groove 110 comes into contact with the tip 111 of the retaining member 113 before the rotor 100 comes off the other components. In this way, the retaining mechanism of the shaft retaining type prevents the rotor 100 from coming off.
The retaining mechanism of the shaft retaining type, however, requires extension of the shaft 105 so as to provide the groove 110 on the lower end side of the shaft 105. The thickness of the spindle motor is inevitably increased by the extended length of the shaft 105. It is, therefore, difficult to reduce the thickness of the data storage medium drive with this structure.
A retaining mechanism of the hook-shaped type includes a retaining member 121 having a spring arm 120 with a tip hook-shaped in cross section, as shown in FIG. 10. The retaining member 121 is attached to the lower surface of the turntable 103 of the rotor 100 with the spring arm 120 projected toward the housing 102. Then, a rotor retaining mechanism 122 is disposed at the top of a tubular portion of the housing 102 inside the spindle motor, with due attention to the length of the shaft not to be shortened. The retaining mechanism of the hook-shaped type thus enables reduction in thickness of the data storage medium drive while maintaining the rotational stability of the rotor, as compared with the retaining mechanism of the shaft retaining type.
If, however, the thickness of the data storage medium drive shall be further reduced, the thickness of the turntable 103 must be reduced. Consequently, the rigidity of the turntable 103 is lowered. In this case, even with the retaining mechanism of the hook-shape type, there arises a problem of possible deformation of the turntable 103 attached with the retaining member 121, e.g., during the assembly of the spindle motor, due to an external impact, or upon the load applied at the attachment or removal of the data storage medium. In order to ensure the rotational accuracy of the data storage medium, the deformation of the turntable 103 must be avoided.